Switched mode power supplies are efficient power supplies that achieve their power efficiency by minimizing dissipative voltage drops with the power supply circuit. Traditional switched mode power supply systems typically use diodes to control and direct and the conduction of current throughout the switched mode power supply. These diodes, however, dissipate power during the diodes' forward current conduction. One technique used to prevent power losses from the forward conduction of diodes is to use synchronous rectifier (SR) MOSFETs. Rather than dissipate power across a diode drop of about 700 mV, an SR MOSFET based power supply system can be designed to have a lower voltage drop, for example, less than 100 mV. In this way, the switched mode power converter's efficiency is significantly improved.
In practical power supply systems, however, SR MOSFET operation should be dependent on load conditions. SR MOSFETs are more efficient than series diodes when the power supply is supplying a nominal or high amount of power. Under light loads, especially at high switching frequencies, switching losses due to the charging and discharging of MOSFET device capacitance may actually exceed conduction losses in corresponding diodes. In such cases, it makes sense to deactivate these SR MOSFETs in order to save power.
In order for a switch mode power supply to be more efficient under high frequency and both high load and low load conditions, a determination needs to be made on whether or not to activate the SR MOSFET switches. One of the challenges of designing such a system is detecting a load condition and making the determination of whether to activate or deactivate the SR MOSFET switches in the power converter circuit.
One way to determine whether to activate the SR MOSFETs is to make a determination based on the turn-on duty ratio of the primary switch. The duty ratio of the primary switch, however, does not always indicate the load level of the power supply directly. For example, at high line or input voltage and certain loads. In this case the duty ratio of the primary switch is much shorter than normal conditions, but shutting off the SR MOSFET devices under low duty cycle conditions in this case would not be power efficient because of high currents running though the switching devices. Other examples of when it is more power efficient to operate SR MOSFETs when the primary duty cycle is low during start up or during overload conditions. The primary disadvantage of using the primary switch turn on duty ratio is that more accurate information about the load level is available from signals present on the secondary side of the power converter.
One possible solution for creating load dependent SR MOSFET operation is to make a real-time current measurement on the secondary coil. One low cost solution that achieves this is using a sense resistor and a valuation voltage across the sense resistor. This solution, however, is disadvantageous because of losses in the shut resistor and the difficulty of measuring current at different load levels.
What are needed are power efficient and low cost circuits and methods that sense output current conditions and use this information to determine whether to activate SR MOSFETs in a switched mode power supply system.